1. Field of the Invention
This invention relates to piston-type internal combustion engines having a single piston and an exhaust heat recuperator which preheats the compressed air charge and passes it into a pre-chamber for combustion. The invention relates further to an engine having a recuperator and a protective valve to protect the recuperator from the combustion process.
2. Description of the Related Art
Internal combustion engines today, with the exception of Diesels, operate on what is commonly known as an Otto cycle originally patented in France in 1862 by Alphonse Beau de Rochas. In 1876, the Rochas compression cycle was incorporated into a practical engine by Nicholas A. Otto. Otto engines were immediately more efficient than Lenoir non-compressing gas engines which had been in production since 1862. Then in 1892, Rudolf Diesel invented the compression ignition engine with higher efficiency than an Otto engine. At the time, their efficiencies were about 3 to 4% for the Lenoir, 12% for the Otto, and 24% for the Diesel, and compared with their expansion ratios of approximately 1.5: 1, 2.5: 1, and 16: 1.
The low efficiencies are related to the large amount of energy remaining in the engine exhaust at the moment of release by the exhaust valve. Exhaust temperatures of 1,450xc2x0 Fahrenheit or more for example were reported for the Lenoir and Otto, and around 900xc2x0 F. for the Diesel. Actual gas temperatures inside the cylinders when expansion was complete were surely much higher. This is because a great deal of heat transfers to the exhaust valve and then to the exhaust port walls. For example, gas reaches about 90% equilibrium with wall temperature after flowing only ten diameters along the length of a straight pipe. In early engines, exhaust valves and ports were labyrinthine in design and thus absorbed much of the heat from the exhaust before it exitted the engine.
The better efficiency of the Diesel came about due to its very high expansion ratio, a result of the high compression ratio needed to create high temperature sufficient to auto-ignite the injected fuel. The high compression ratio and attendant gas and bearing pressures required greatly increased strength and with it, increased weight and cost. In fact, the Diesel is two to three times the weight and cost of a comparable Otto engine.
In present day Otto engines such as those used in automobiles, compression/expansion ratios are typically around 8:1 and expansion of the combustion gases is far from complete. Thus, at full load, exhaust is released at 90 to 120 pounds per square inch and 2,500 to 3,200xc2x0 F. and carries away to the coolant and the atmosphere nearly half of the input fuel energy.
A partial solution to energy wastage with the exhaust has been the turbo-expansive conversion of exhaust energy to rotative energy, the rotative energy then being used to drive a turbo-compressor for boosting input air pressure to an engine. Turbines however cannot tolerate direct exhaust heat from an Otto engine nor are they suited to the pulsating exhaust flow from a single cylinder. Exhaust from several cylinders is merged to smooth the flow and passed through exposed pipes to dump part of the heat to the atmosphere so as to cool the gases by 1,000 to 1,500xc2x0 F. Exhaust turbines have thus been only minimally effective in raising the efficiency of Otto engines.
Recuperative gas turbines were developed for stationary use beginning around 1950 wherein residual exhaust heat is captured and returned to the compressed air before it enters the burners. Gas turbines operate on the Brayton gas cycle and differ from the Otto cycle in that combustion takes place at constant pressure and that expansion continues typically back to atmospheric pressure. Brayton and Otto cycles are similar however in that the exhaust exitting from a gas turbine is usually at a higher temperature than the compressed air entering the combustion chamber. The opportunity thus exists for a reduction in the fuel input required, since the heat being exhausted can be used (at the cost of a heat exchanger and connecting ducting) to provide part of the heat input, with consequent increase in efficiency. A brief review of the regenerative gas-turbine cycle may be found in Engineering Thermodynamics by M. C. Potter and C. W. Somerton, McGraw-Hill, 1993. A more complete analysis of recuperative gas turbines may be found in Marine Gas Turbines by John B. Woodward, John Wiley and Sons, 1975.
Gas turbines with heat exchangers are under development today for military use as evidenced by a request for proposal published in the Department of Defense Fiscal Year 1998 Small Business Innovation Research (SBIR) Program #A98-013 Titled: Advanced Ultra Compact Heat Exchangers. In this request, it is noted that present day, state-of-the-art recuperated gas turbine engines use metallic heat exchangers which are exceedingly heavy and larger in volume than all the engine turbomachinery components combined, thereby precluding their use in air vehicles.
In the prior art, recuperative engines have generally had adequate recovery of exhaust heat but their transfer of heat to the working charge has been inefficient. This is because the high temperature, high grade thermal energy available in and recovered from the exhaust has been allowed to degrade prior to its transfer to the working charge. Early inventors of heat engines sought effective use of recuperators, but often compromised thermal efficiency by reducing temperatures either to protect working materials and surfaces or to avoid problems with detonation or pre-ignition in the combustible mixture.
The first known recuperative internal combustion engine of the prior art is described in U.S. Pat. No. 155,087 granted Sep. 15, 1874 to Joseph Hirsch. It has two cylinders in a vee plus an air pump adjoined at their working ends by a duct containing a regenerator and is described as a hot-air engine.
In U.S. Pat. No. 328,970 granted Oct. 27, 1885, inventor James F. Place describes a recuperative engine that uses two stage, counter-flow transfer of exhaust heat to the compressed charge. Place used two cylinders approximately 70xc2x0 apart in phase to provide separate compression and expansion in his engine.
Looking now at the art related to single piston recuperative engines, U.S. Pat. No. 1,190,830 granted to J. F. Wentworth describes a single cylinder engine having transfer of exhaust heat to the incoming air charge. Since the transfer is prior to compression, the engine cycle is not advantageously recuperative. Wentworth recognized at an early date, the utility of a thermally insulating liner in the combustion chamber and on the piston cap to reduce heat losses from combustion. The refactory metal liner was separated by a space from the cylinder head and the space filled with a refractory insulation such as asbestos.
U.S. Pat. No. 1,945,818 granted to H. L. McPherson and J. W. Weatherford describes an engine with no recuperator but having a separated combustion chamber connected by a duct to a single cylinder with a reciprocating piston therein, the piston being shaped to provide minimum clearance with the cylinder head. Fuel input and a spark plug provide for combustion within the chamber while a poppet valve controls gas flow through the duct between the chamber and the cylinder. The poppet valve is perforated by holes 23, believed to be for cooling.
U.S. Pat. No. 2,671,311 granted to H. Rohrback is similar to the engines of some of the earlier patents in the use of a liquid coolant, in this instance, by injection of coolant into the cylinder after the work stroke to cool the cylinder and piston. A condenser for the volatized liquid coolant acts as a heat exchanger to heat air being drawn therethrough for injection into the cylinder by a supercharger. The Carnot efficiency of this engine would not be significantly improved since the cycle is not recuperative.
U.S. Pat. No. 3,591,958 granted to William H. Nebgen and assigned to Treadwell Corporation, teaches the use of an external turbo-compressor to add chilled compressed air to the single cylinder of an internal combustion engine for the expressed purpose of cooling the internal parts of the engine including the piston and cylinder. At the end of the compression stroke, the compressed air is expelled from the cylinder and enters a manifold to pass through a heat recuperator. After the compressed air has been returned to the cylinder, combined with fuel, ignited and expanded in the cylinder to perform work on the piston, the exhaust is expelled to an exhaust manifold, passed through two heat recuperators in series and then to the atmosphere. Nonetheless, this hot exhaust gas is then fed back into the cylinder for admixture with additional cold, compressed air. The recuperators do not appear to be intimately associated with the working cylinder and would probably have large radiation and convection heat losses. Additionally, energy must be expended to operate the refrigerator and the turbo-compressor, which energy is not fully recoverable. As a consequence, the thermal efficiency is less than optimum.
U.S. Pat. No. 4,280,468 granted to Millman and four patents including U.S. Pat. No. 5,632,255 granted to Ferrenberg describe a single cylinder engine having a moving plate recuperator with diameter equal to the piston and including a mechanism to mechanically reciprocate the recuperator in the cylinder between the piston and the cylinder head. While this engine offers improved thermodynamic efficiency, the recuperator is located within the combustion space and exposed to combusting gases.
U.S. Pat. No. 4,284,055 granted to Anthony C. Wakeman and assigned to Lucas Industries, Limited, of Great Britain proposes to provide a low compression ratio reciprocating piston internal combustion engine wherein exhaust heat is recovered in a recuperative heat exchanger located within the working volume of the cylinder above the top dead center of the piston and below the cylinder head. With this arrangement, it again appears that the recuperative heat exchanger is directly exposed to combusting gases. Wakeman uses an additional piston to displace gases through the recuperator.
There is of course another group of engines, generally called Stirling engines, which have recuperators for internal gas exchange. This art is considered unrelated since the engines have external combustion and the temperature at which the recuperator operates is considerably lower than in the cited art.
The problem with most of these cited internal combustion engines employing a heat exchanger or recuperator of one type or another is the large radiative and convective heat loss caused by the exposed location and/or the large size of the heat exchanging element. As pointed out in the discussion of individual patents, heat losses from the recuperator lower the Carnot efficiency. In the very few instances in the prior art where the recuperator is not subject to radiation and convection losses, the recuperator is located in the working cylinder or in a duct directly connected with the working cylinder and the recuperator is thus directly exposed to the flame front of the ignited charge.
Intermediate between the Otto and the Diesel there has remained the possibility of an engine type which would extract more energy from the combustion gas but without the weight and cost penalties of the Diesel. A direct approach has been to capture heat from the exhaust and put it back into a subsequent engine cycle. This process of recovering heat has been referred to in the prior art as regeneration, heat exchanging, heat recycling and recuperation. The latter term will be used herein. Recuperation is believed to be effective only when the recovered heat is put into the charge after the charge is trapped in the working chambers of the engine. The term xe2x80x9crecuperative enginexe2x80x9d will also be reserved in this specification for those engines which have positive displacement, an actual throughput of combustion gas, and combustion taking place internally within a working chamber.
A recuperative or heat exchanging cycle, referred to hereafter as the Hx Cycle (pronounced xe2x80x9cwixxe2x80x9d), will be used herein to refer to an engine cycle wherein heat captured from the exhaust is used to heat a subsequent compressed charge.
The maximum efficiency that a heat engine such as the Otto Cycle engine can have is calculable by Carnot""s Law. This law states:       Maximum  possible  efficiency    =      η    =          1      -                        T          Low                          T          High                    
THigh in the above equation is the combustion temperature, and TLOW is the exhaust temperature.
The maximum possible efficiency of the Hx Cycle, where DTrec is the temperature reduction achieved in the engine exhaust by the recuperator, is:       Max.  possible  recuperative  efficiency    =      η    =          1      -                                    T            Low                    -                      Δ            ⁢                          xe2x80x83                        ⁢                          T              rec                                                            T            High                    +                      Δ            ⁢                          xe2x80x83                        ⁢                          T              rec                                          
Putting in typical temperatures in degrees Rankine for an Otto engine at peak output, but with recuperation, we get:       Maximum efficiency    ,      η    =                  1        -                  (                                                    3                ⁢                                  ,                                ⁢                000                            -              600                                                      5                ⁢                                  ,                                ⁢                500                            +              600                                )                    =      .606      
For the Otto engine without recuperator, the efficiency maximum is:       Maximum efficiency    ,      η    =                  1        -                  (                                    3              ⁢                              ,                            ⁢              000                                      5              ⁢                              ,                            ⁢              500                                )                    =      .455      
Six hundred Rankine degrees of recuperation, the amount obtained in tests of a prototype, thus raises the ideal efficiency from 45.5% to 60.6%. It must be understood that these numbers are good only for comparative purposes and in actuality, one may obtain only around one-half of their value, the Otto engine today providing about 22 to 25% efficiency in automotive use. Many factors have not been accounted for such as heat losses to the combustion chamber and recuperator walls, gas flow losses due to resistance to compressed charge flow through the recuperator into the combustion chamber, and to exhaust flow from the expander cylinder out through the recuperator to atmosphere, and the variation of specific heat of air and combustion gases with temperature, which variation necessitates a correction factor for DTrec. making it larger when below and smaller when above its mean value.
The problems of Otto engines and known recuperative engines of the prior art are solved by my invention wherein I provide a new and novel cylinder head which provides a working four-stroke cycle unit over each piston of an engine. The cylinder head contains an internal recuperator for heating the compressed air charge which air is then fed into an internal combustion chamber (also known in the art as a pre-chamber), admixed with fuel, and combusted. Life shortening of the recuperator through exposure to the flaming fuel-air mixture has also been eliminated by the use of a recuperator protective valve. The recuperator protective valve closes to separate the recuperator from the combustion chamber just prior to combustion. It reopens at the end of the expansion stroke to release the exhaust through the recuperator to atmosphere. The protective valve is needed in all but the smallest, lowest output engines to protect the recuperator from damage from the combustion flame front.
This arrangement of combustion chamber and compact recuperator obtains improved Carnot efficiency in a simple, cost effective manner, the heat losses caused by radiation and convection being reduced due to the recuperator having a volume on the order of only 5% of the cylinder displacement. Thermal losses are then reduced further by the use of a novel insulating liner around the recuperator and combustion chamber. In a preferred embodiment, a separated duct recuperator built around the exhaust valve provides further enhanced efficiency whereby a recuperative exhaust valve may be built as an integral unit.
In larger engines, residual energy still in the exhaust after it leaves the recuperator (perhaps as much as 25% of fuel energy), may be partially recovered with an exhaust driven turbo-alternator to produce electric energy. Since most engine applications already use an alternator to provide electric power, capture of alternator drive energy energy from the recuperative engine exhaust can take the place of the present alternator system and further improve efficiency.
The recuperative cylinder head may be engineered to match existing conventional engine blocks such as are used in modern gasoline fueled automobiles, thereby reducing the cost of changeover to new production. In addition, the head may be engineered to retrofit to the block of engines which originally had drive means such as a notched timing belt for operating overhead valves.
In this invention, an exhaust heat recuperator extracts heat from the exhaust, thereby lowering the heat rejection temperature and subsequently adds this heat to the compressed working charge prior to combustion, thereby raising the pre-combustion temperature which produces an equivalent increase (after adjustment for the change in specific heat) in the peak combustion temperature.
A first object of this invention is therefore to maximize, through the method of recuperation, the potential Carnot efficiency and the related actual working efficiency of a single piston internal combustion engine.
Another object is to provide effective recuperation in an engine with the minimum number of additional valves, pistons, recuperators, and other ironmongery, which in the preferred embodiment is reduced to a single reciprocating piston and four valves per engine unit.
A third object of this invention is to provide a coaxial, symmetric method of construction for a combustion chamber, recuperator, valving, and insulative housing all within a common cylinder head.
Another object is to provide a means for insulating a combustion chamber and recuperator with a simple mechanical construction that due to symmetry is resistant to thermal stress fracture and temperature non-uniformities.
Another object of the invention is to provide for an engine, a recuperator unit made in thermally segregated sections with reduced axial heat flow loss.
Another object is to provide a combination recuperator and exhaust valve in an assembly having separated parallel coaxial ducts comprising an outer one which carries exhaust from the combustion chamber out of the engine in a first direction, and an inner duct which carries compressed air in an opposite direction into the engine combustion chamber.
Another object is to provide a common duct recuperator having a tapering cross section along it flow axis to compensate for the changing viscosity of gases with temperature.
Yet another object is to provide a separated duct recuperator having tapering cross sections along its flow axes, the hotter ends of each duct having larger area, to compensate for the increasing viscosity of the gas flows with temperature.
Another object is to provide a method for installing a recuperator such that it may be easily removed for servicing, much like a spark plug.
Yet another object is to provide a recuperative cylinder head for easy attachment to new engine blocks and also easy retrofit to existing conventional engine blocks to replace conventional Otto Cycle cylinder heads.